In the inventory if you filled up all 42 spots than currently there is not a way to get more space.However you may make use of the Warehouses provided to you,this is the other alternative at this moment.

Inventory
Warehouse
Account Warehouse
Guild Warehouse
2 more characters on the account each with another inventory and warehouse page (could go crazy and make guilds just for them for another guild storage)
And then you also have the option to trade with people you TRUST to another account for even more storage.

Outside of that, the only option to expand inventory further may have to be bought from the Item Mall.. but there's no date set for that and I don't remember seeing it in the default items in there currently.